Initially, negative acting photoresists were applied as liquids to copper substrates and radiated imagewise to form relatively insoluble material in the exposed areas and left relatively soluble material which was washed away upon treatment with a developer to reveal the copper substrate in the non-exposed areas. The copper was then etched away and the relatively insoluble material covering the remaining copper traces was stripped by a stronger solvent.
The processing of positive acting photoresists generally follows the sequence of applying a solution of the resist to a copper foil laminated to an epoxy resin base, drying and baking the resist to expel the solvent, exposing the resist to actinic radiation through a patterned photomask to define an image, dissolving the exposed portions of the resist in an alkaline developer to delineate the image, rinsing, and in some instances post-baking the imaged resist. The areas void of resist, comprised of exposed copper substrate, are then either etched or electroplated. If the exposed copper is etched, the unexposed portions of the resist which cover the remaining copper foil may then be stripped away by a more active alkaline solvent to reveal the desired pattern of copper traces. If the copper exposed during development is electroplated with gold, for example, the newly exposed in the stripping step is then removed by etching.
The early use of a laminatable, negative acting single layer film on a substrate to make a dry film photoresist is discussed in U.S. Pat. No. 3,469,982. That was followed a few years later by the use of laminatable films of a positive acting compositions to make dry film photoresists as disclosed in U.S. Pat. No. 4,193,797.
Particular requirements in the positive acting photoresist art, however, have led to the use of two layer or multilayer laminatable sheets of photoresist materials in order to make dry films having improved sensitometric and physical properties.
An oleophilic, moderately flexible dry film photoresist comprising two intermixed and intertwined resin networks and a positive acting photosensitizer is described in 3M's U.S. Pat. No. 4,247,616. The first network comprises a cross-linked urethane derived from a novolac resin and a polyisocyanate compound; the second is a heat curable epoxy resin plus a curing agent therefor. The photosensitizer generates acidity in the irradiated areas.
Although, the dry film of the 3M patent is a monolayer, a later 3M patent, U.S. Pat. No. 4,672,020, teaches that positive acting dry film photoresists having the monolayer construction could not meet the then existing commercial needs for resistance to crazing during storage, photosensitivity, and thermal dimensional stability. The solution to the problems taught by the later 3M patent is a positive acting multilayer dry film photoresist comprising at least two functional layers and, optionally, a strippable carrier layer. The first functional layer comprises an o-quinone diazide in a phenol formaldehyde resin binder which becomes more soluble in an aqueous alkaline solution upon exposure to actinic radiation. The second layer, comprising an acrylic copolymer or terpolymer and a phenol formaldehyde resin crosslinked with a polyisocyanate, is adjacent and adhered to the first layer.
The performance limitations exhibited by typical negative acting dry film photoresists include:
1. resolution limitations, limited aspect ratio; PA1 2. resist and debris related defects in the manufacture of innerlayers which lead to opens, near opens, and pits; PA1 3. resist and debris related defects in the manufacture of outerlayers which lead to short circuits, reduced spacing, and under plating; PA1 4. polyethylene coversheet required to allow for roll packaging of the photoresist composition; PA1 5. limited process latitudes (exposure, development); and PA1 6. leaching of organics during plating cycles. PA1 UV-transparent resin binder system allows for efficient photobleaching of the photoactive component, affording lower exposure doses; PA1 flexible, tack free, excellent tensile strength; PA1 does not crumble or flake when handled; PA1 does not stress crack when tightly rolled up or folded; PA1 adheres to a carrier sheet such as polyester prior to lamination; PA1 adheres to copper clad dielectric using conventional lamination techniques; PA1 the solvated resist formulation may be directly applied to copper clad dielectric and dried, thus resulting in a photosensitive laminate for which the resist coating exhibits tack-free adhesion to the substrate and flexibility; PA1 shelf-life of at least one year; PA1 variations on the resist formulation enable vacuum lamination capability; PA1 exhibits little to no odor at room temperature or when heated during lamination; also is relatively non-toxic; PA1 no cold flow or telescoping; PA1 uses conventional lamination equipment, pressures, and temperatures; PA1 uses conventional exposure equipment and light sources; PA1 conventional development equipment, chemistries, temperatures, spray pressures, concentrations, and durations used for negative photoresists may be employed; PA1 capable of on-contact patterned exposure using irradiation doses in the realm of current negative acting dry films without evolution of nitrogen during or after exposure; not sensitive to oxygen; PA1 capable of multiple exposures, e.g., exposure of unexposed areas after a plating cycle; PA1 attenuates the need for artwork compensation or biasing commonly used with negative acting dry films; PA1 exhibits one-to-one linearity with the phototool used; PA1 more tolerant to overexposure as compared to negative acting dry films, even from non-collimated light sources; PA1 straight or slightly sloped sidewalls after aqueous alkaline development attenuating the resist foot typically observed with negative acting photoresists; PA1 space growth (the inverse of line growth exhibited by the negative acting dry films) occurs only when gross off-contact exposures are performed; PA1 wide exposure latitude; less sensitive to scattered or stray light sources; PA1 excellent resolution (aspect ratio) and adhesion properties; PA1 permits circuit densification (finer etched spaces, finer conductor widths), and landless vias; PA1 minimal (&lt;5%) unexposed film loss after aqueous alkaline development; PA1 excellent gloss and abrasion resistance after aqueous alkaline development and after acid etching; PA1 less sensitive to over development than negative acting dry film photoresists; PA1 opens, near opens, pits in the manufacture of innerlayers minimized; PA1 short circuits, reduced spacing, under plating in the manufacture of outerlayers minimized; PA1 conventional equipment, chemistries, temperatures, spray pressures, concentrations, and durations used in acid etching; PA1 conventional equipment, chemistries, temperatures, concentrations, and durations used in the electrochemical plating of metals such as copper, tin, nickel, and gold; and PA1 a lesser number of ingredients relative to typical negative acting dry film photoresist compositions, thereby lowering the standard cost and overhead required to test and stock raw materials; PA1 no leaching into chemical plating baths during plating cycles.
For most circuit board manufacturers, the use of positive acting liquid photoresists in the fabrication of printed circuit boards has been a financially difficult proposition because of the capital expenditure required to purchase, set-up and maintain an electrodeposited photoresist system, a liquid photoresist roller coater, drying oven and ancillary cleanroom.
Another performance limitation of positive acting liquid photoresists in the fabrication of printed circuit boards has been artwork tenting during on-contact patterned exposure. The transporting of panels coated on both sides with liquid resist through the drying oven requires that each panel be suspended by means of a mechanical gripping system. Thus, each panel is held in areas where resist has already been coated. As a result, liquid resist is transferred onto the conveyor mechanism itself. Over time this leads to airborne, resist debris within the confines of the drying oven. This debris then deposits on subsequently coated panels, while still wet, leading to debris laden dry resist coatings. During on-contact exposure, this embedded debris can lead to artwork tenting resulting in light scatter and defocus problems.
The slow photospeed of positive acting liquid photoresists compared to that of negative acting dry films, even when thin coatings (e.g., 0.5 mil) are applied, has been still another of their performance limitations.